Gravity wave characteristics in the middle atmosphere derived from the Empirical Mode Decomposition method

The recently developed Empirical Mode Decomposition (EMD) method is applied to analyzing gravity wave characteristics in the middle atmosphere. By establishing a close connection between the fundamental Intrinsic Mode Functions (IMFs) derived from the EMD method and WKB solutions of a dispersive-dissipative wave equation, we show that the EMD method can provide useful insights into physical processes in the middle atmosphere where dispersive-dissipative wave phenomena are dominant. A local power spectrum function P is introduced which provides a quantitative description of the spectrum at any particular location within a data series. The sharp localization of P in space and wavenumber leads to an identification of unphysical small scale oscillations by falling spheres embedded in the wind profiles above 60 km. Further analyses of the horizontal wind profiles derived from the Dynamics Adapted Network for the Atmosphere (DYANA) campaign suggest that for horizontal wind fluctuations with vertical wavenumber m less than or equal to 3 km(-1) (or vertical wavelength L-z greater than or equal to 2 km) the previously observed m(-3) Fourier spectra could be produced by a linear wave packet whose characteristic vertical decreases with altitude. For small vertical scale disturbances with m > 3 km(-1) (L-z < 2 km) a near -3 slope in the marginal distribution exists locally in the middle atmosphere with a great degree of universality, suggesting that nonlinear energy cascade processes may dominate the spectral formation in this wavenumber range.